It depends on what type of business you are going to do. Is the press going to be in one spot (i.e. stationary)? Do you plan to do event productions (i.e. fairs, flea markets, sport tournaments, ...)? How much space do you have for this piece of equipment (i.e. clamshell presses take up less space than a swing-away press)?
A 16" x 20" is always best because it allows for shirts 2 XL and larger where you want the design to be longer than 14.5" (will want to leave some space all the way around the platten to make sure that the heating element covers the design completely). However, these presses usually are pretty heavy and are not the easiest to pick up and move around for event production. For events, you might want to look at the Hix Swingman. It has manual controls for heat and time, but it is pretty light and portable.
Imprintables Warehouse has one of the best clamshells out on the market - Phoenix Phire for the following reasons, but it is not the cheapest:
1. Automatic Release Technology that opens the press when the timer expires
2. Magnetic assistance when closing the press to make it easier on the worker
3. Center pressure adjustment knob above the heating element.
4. Floating heating element to allow for some adjustment on no flat items (i.e. hoodies)
5. All the metal pieces are solid and are not welded. This makes it much stronger down the road.
The automatic release function is great because it allows you to do multiple things and not worry about the shirt burning. Geo Knight offers an automatic release function as an after-market attachment. So, if you can't afford this function now, you can add it on later.
The best thing you can do is go to a trade show or a distributor and really see first-hand the benefits of the different styles of presses. Then, find the one that works best for your business now and at least a couple years down the road. Heat presses usually last a long time unless you buy one of the off-brands that are produced overseas and are being sold for pennies. Stick to the Hix, Geo Knight, Insta, Hotronix and iDek presses. Also, check out eBay and you could get a deal. Good Luck.
Mark